After a three-day rescue effort, the fate of 40 pilot whales hung in the balance, with officials saying they had lost track of two dozen of them in southern Florida.

The search has been halted on Friday due to darkness and weather constraints but it will resume at first light on Saturday with a helicopter scanning the waters.

"Anything could happen, whales could change direction, flight going up in the morning to locate whales again," an official National Oceanic and Atmospheric Administration account tweeted.

The agency is coordinating the search.

With 11 whales already dead, a second NOAA tweet said biologists were concerned to hear of behavior including dehydration and malnutrition among whales being monitored.

NOAA marine mammal scientist Blair Mase suggested it was "encouraging" that 24 of the mammals had not been located.

We are "hoping they are out to sea," Mase told reporters in a conference call. But she acknowledged she was "not certain" the missing group had made its way out of difficult, shallow waters to safety.

Mass stranding

The mysterious mass stranding in a remote area off the US state's Everglades National Park was first reported on Tuesday. Some of the whales are unaccounted for amid fears they died and sank to the ocean floor.

Of the 11 that perished, four had to be euthanized after they became unable to breathe, hydrate and feed in the shallow waters less than a meter deep off Highland Beach on the Gulf Coast.

Scientists have performed necropsies on all of the dead whales to look for possible diseases and pathogens as well as environmental and human causes of death. But no such problems have yet been detected.

Of the remaining whales that have been sighted, seven were in dangerously shallow waters, about eight km offshore and southwest of where they were originally stranded, Mase said.

Another two were "swimming freely" around Plover Key, she said, adding that a biologist was on his way to the area to assess their condition.

While a preliminary NOAA assessment put the total number of stranded whales at 51, officials said Friday they did not have an exact figure.

On Thursday, a rescue team of experts from several agencies, including the National Park Service and the US Coast Guard, grew from 25 to 35 scientists and stranding specialists who were only able to reach the whales by boat.

But with the clock ticking, NOAA has stressed that the rescuers were treading cautiously as sharks have been seen feeding on the dead whales' carcasses.

It was still unclear why the whales swam to the warm, shallow waters.

In the absence of concrete findings, some experts and animal welfare organizations speculated that possible disorientation after a maritime explosion or a disorder caused by environmental pollution that leads to suicide may have played a role.

Rescuers began their efforts Tuesday after park rangers were told that four whales were stranded. When teams arrived at the scene they discovered a much larger number.

But their endeavors - and the subsequent monitoring of the mammals - were complicated by the fact that the beaching occurred in a very isolated part of southern Florida where there is no cell phone reception.

Everglades National Park provides a unique habitat for a range of species, including crocodiles.

Pilot whales are very cohesive among themselves, and usually travel in big family groups.

The mass beaching is not a first. Last year, more than 20 pilot whales were stranded in the Florida Keys. In September, just five of a group of 20 survived after they, too, lost their way.